[leetcode]144. Binary Tree Preorder Traversal -- JavaScript 代码

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var preorderTraversal = function(root) {
    var ret = [];
    if(root === null){
        return ret;
    }
    var stack = [];
    var cur = root;
    while(cur!==null){
        ret.push(cur.val);
        left = cur.left;
        right  = cur.right;
        if(left !== null && right !== null){
            stack.push(right);
            cur = left;
        }else if(left === null && right === null){
            if(stack.length>0){
                cur = stack.pop();
            }else{
                cur = null;
            }
        }else if(left === null){
            cur = right;
        }else{
            cur = left;
        }
        //console.log(cur);
    }
    return ret;
};

这道题用递归可以很好的解决，可是题目说是否可以不用递归。

于是就只好利用栈来完成了。

思路也很清晰。